A GLIMPSE into the very faint-end of the H$\beta$+[OIII]$\lambda\lambda$4960,5008 luminosity function at z=7-9 behind Abell S1063

Abstract

We use the ultra-deep GLIMPSE JWST/NIRCam survey to constrain the faint-end of the H$\beta$+[OIII]$\lambda\lambda$4960,5008 luminosity function (LF) down to $10^{39}$ erg/s at z=7-9 behind the lensed Hubble Frontier Field Abell S1063. We perform SED fitting on a Lyman-Break Galaxy sample, measuring combined H$\beta$+[OIII] fluxes to construct the emission-line LF. The resulting LF ($\alpha$=-1.55 to -1.78) is flatter than the UV LF ($\alpha<-2$), indicating a lower number density of low H$\beta$+[OIII] emitters at fixed MUV. We explore three explanations: (i) bursty star formation histories reducing the H$\beta$+[OIII]-to-UV ratio, (ii) metallicity effects on [OIII]/H$\beta$, or (iii) a faint-end turnover in the UV LF. Assuming an evolving [OIII]/H$\beta$ ratio, we derive a flatter [OIII]$\lambda$5008 LF ($\alpha$=-1.45 to -1.66) and a steeper H$\beta$ LF ($\alpha$=-1.68 to -1.95). The combination of decreasing metallicity and bursty star formation can reconcile the UV and H$\beta$+[OIII] LF differences. Converting the LF to the ionising photon production rate, we find that galaxies with H$\alpha$ flux $>10^{39}$ erg/s (SFR(H$\alpha$)$>5\times10^{-3} M_\odot$/yr) contribute 31%-90% and 46%-156% of the ionising photon budget at 7<z<8 and 8<z<9, respectively (for $f_{esc}=0.14$). The LF shape suggests faint galaxies contribute minimally to the ionising photon production rate. Our cosmic star formation rate density (CSFRD) estimates align with previous work, but GLIMPSE's sensitivity to low SFRs confirms that very faint galaxies are minor contributors to both the ionising photon production rate and the CSFRD. Our results suggest that GLIMPSE has detected the bulk of the total H$\beta$+[OIII] emission from star-forming galaxies, with fainter sources playing a limited role in cosmic reionisation.